Water, water everywhere and not a drop to put into my equilibrium constant calculation...

Kc, Kw, Ka, Kp...

When do you add water to the equation and when do you not?

This is more straight forward than you think.

It comes down to how much water have you got. If your whole reaction is happening in water then you have got so much of it that the little bit that you are making is irrelevant and should be left outt of the calculation of Kc. Effectively it comes down to the fact you can't dissolve water in water.

So any aqueous reaction you leave out water, this is most reactions including all Kw and Ka's and most Kc's.

There will be some Kc calculations where you are reacting liquids and you deliberately don't add water. Esterification is the most obvious of these. To get the reaction to work you make sure water is not present (as it is a product and the equilibrium will stay to the left). There are very few others.

Kp is different as this is all about pressure and water, just like everything else, has a pressure as long as it is a gas.

What's the point of gritting the roads?

Have you ever wondered why we grit the roads? I know the obvious answer here is because it makes the ice melt, but why?  Why does gritting the road cause the ice to melt? Well, here is the answer to that question on a molecular level.

Grit (or Halite) is sodium chloride with all the impurities from mining left in. When it is added to ice It causes the freezing point of water to be lowered. So at, say -5C, pure water is solid but water with a touch of NaCl is a liquid.

This is all to do with equilibria, imagine a puddle that's freezing, solid water in the puddle is in equilibrium with the liquid water in the puddle and as freezing is an exothermic process at this low temperature freezing is favoured over melting and the water freezes.

Adding salt to the liquid water means that you are effectively "reducing the concentration of liquid water" because now it is not pure water it is water and salt. What difference does make to the equilibrium? Well, reducing concentration of a reactant favours the production of that reactant so liquid water is favoured over solid water and ice isn't formed.

Is this a special property of ice? Nope, any substance that dissolves in water will do the job, it's just that grit is cheap.

This also explains another weird effect. If you add salt to ice the temperature drops significantly, it can get down to -10C. This is because melting is favoured and melting is endothermic so the process of melting removes heat from the surroundings and drops the temperature.

Feel free to bore your friends with this explanation every time a gritter drives past over Easter.